Selective upregulation of the flip-flop splice variants of AMPA receptor subunits in the rat spinal cord after hindpaw inflammation

Glutamate receptors are involved in spinal nociceptive transmission and the development of persistent inflammatory hyperalgesia. It is unclear, however, whether there are changes in glutamate receptor gene expression associated with tissue injury. In the present study, we used reverse transcription-polymerase chain reaction (RT-PCR) to examine the modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor gene expression in the rat spinal cord by inflammation. Inflammation was introduced into the hindpaw by intraplantar injection of 0.2 ml of complete Freund's adjuvant (CFA). At 2 h-14 d after inflammation, total RNAs from L4,5 spinal cord were used for RT-PCR with primers targeted at eight flip-flop splice variants of the AMPA receptor subunits. It was found that the GluR1-flop mRNA was up-regulated at 2 h-5 h (P<0.05), down-regulated at 3 d (P=0.05), and returned to control levels at 7 d following inflammation. The GluR2-flip and GluR3-flop mRNAs were up-regulated at 5 h-1 d (P<0.05) and returned to control levels at 3 d after inflammation. The GluR1-flip mRNA was not detected in the samples and the mRNAs for other splice variants did not exhibit significant changes. Immunocytochemical analysis of GluR1 and GluR2 subunits indicate that the protein translation products of these subunits were also increased in the spinal cord. These results demonstrate an increased expression of AMPA receptor subunits that correlates with the acute phase of CFA-induced inflammation and hyperalgesia. Selective changes in the expression of the flip-flop splice variants of the AMPA receptor suggest a reorganization of the composition of the AMPA receptor complex and its involvement in the development of inflammatory hyperalgesia.